This invention relates in general to phase detector apparatus and more particularly to a high gain zero offset linear phase detector for use in frequency synthesizers.
A frequency synthesizer is a device which produces a plurality of synthesized output frequencies where each output frequency is locked to or derived from a stable master frequency source such as a crystal oscillator, an atomic clock and so on. The synthesizer finds many uses in automatic test and communications equipment as for example as a local oscillator for radio receivers and in general as a precision tunable source. The frequency synthesizer has received widespread use and most modern day frequency synthesizers utilize phase locked loops (PLL) as an element in the synthesizer. The PLL includes a phase detector, a low pass filter and a voltage controlled oscillator (VCO). The phase detector is a linear device and its operating characteristics determine the overall loop performance of the phase locked loop.
The term PLL refers to a feedback loop in which the input feedback parameters of interest are the relative phases of the waveforms. The function of a phase detector is to track small differences in phase between the input and feedback signals and to measure the phase difference between two inputs. The output of the phase detector is then filtered by means of a lowpass filter and applied to the control terminal of a voltage controlled oscillator (VCO). The VCO provides an output frequency which is a function of the control signal applied to its control terminal. In the PLL the VCO frequency changes in the direction that reduces the phase difference between the VCO and the reference signal. Such a loop is said to be in phase lock or locked when the phase difference is reduced to zero.
Since the frequency synthesizer is utilized as indicated above in communication and test systems, the output available from the frequency synthesizer must be accurately specified and controlled. One is concerned with frequency range, frequency resolution, frequency indication, frequency error, settling time, output power, harmonic distortion, phase noise, spurious interference, wideband noise and so on. Many of these factors are also associated with the phase locked loop. In particular the phase detector which exists in the phase locked loop is an important component of the loop and under many circumstances can contribute to the purity of the synthesizer output as well as substantially contributing to noise generated by the synthesizer.
Essentially, linear sampling phase detectors are employed in high performance frequency synthesizers to provide improved noise and spurious output performance. These exhibit superior operation than that achievable in synthesizers using digital phase/frequency discriminators.
In a phase detector the primary objects are enable a high speed switching operation and provide improved synthesizer outputs with reduced output spurious frequencies, reduced reference sidebands and lower levels of output phase noise.
It is an object of the present invention to provide an improved sampling phase which sampling phase detector provides lower noise and sideband levels than those in the prior art.
It is another object to provide a linear sampling phase detector with zero offset and which phase detector provides increased gain over prior art phase detectors.